Research
FNR: a global anaerobic transcription factor
FNR, which is a global regulator of gene expression under anaerobic conditions, serves as a paradigm for oxygen sensing. The activity of FNR is regulated by Fe-S cluster destruction via the oxygen dependent removal of a [4Fe-4S] cluster that is required to maintain an active conformation of this protein. Our current efforts are focused on defining the FNR intermediates that form during transitions between aerobic and anaerobic conditions, studying the role of FNR proteolysis, and determining how the presence of the Fe-S cluster promotes protein conformational changes that are required for dimerization and site-specific DNA binding by FNR.
We are also interested in the role of transcription activation domains of FNR in the global control of gene expression. In particular, we are studying the protein determinants that facilitate FNR's interaction with RNA polymerase. In addition, we have identified new members of the FNR regulon from global gene expression studies in collaboration with Dr. Fred Blattner (Genetics). FNR homologs are found in a variety of bacteria, including many pathogenic bacteria, where they play a key role in the metabolic adaptation to an anaerobic lifestyle. Thus, our analysis of E. coli FNR will continue to provide key insights into conserved global regulatory mechanisms for sensing and adapting to changes in oxygen tension that are used by a wide variety of bacteria.
IscR: a negative regulator of Fe-S cluster biosynthesis
IscR, a newly discovered repressor of the Isc Fe-S cluster biogenesis genes, is regulated by cluster synthesis. We found that the genes encoding proteins facilitating Fe-S cluster biogenesis (IscS, IscU, IscA, Hsc66, Hsc20, and ferredoxin) are repressed by IscR through a novel autoregulatory mechanism. IscS, IscU, IscA, Hsc66, Hsc20, and ferredoxin are conserved in organisms from bacteria to humans, and in many bacteria, the genes are organized in an operon, iscSUAhscBAfdx.
Our studies show that expression of these genes is controlled by a negative feedback loop mediated through IscR. IscR is encoded by a gene located immediately upstream of iscS and we found that it contains a [2Fe-2S] cluster. The repression function of IscR appears dependent on the Fe-S cluster assembly proteins, providing a mechanism by which the Fe-S cluster assembly can be coupled to cellular requirements for synthesis or repair of Fe-S proteins. Current work is focused on elucidating the mechanism of IscR repression, and studying how the activity of this protein is modulated. In addition, we are carrying our global gene expression studies to determine what other genes are coordinated by these Fe-S regulatory proteins.